Nitrate reduction on the surface of bimetallic catalysts supported by nano-crystalline beta-zeolite (NBeta)

Abstract

Nitrate contamination of surface and groundwater is one of the most serious environmental issues due to its adverse effects on human health and the ecosystem. In this study, novel bimetallic catalysts supported by nanocrystalline beta zeolite (NBeta) were firstly developed for the complete and selective reduction of nitrate to environment-friendly nitrogen. The synthesis of a NBeta support and bimetallic catalysts was confirmed by X-ray diffraction and Fourier transformation infra-red spectroscopy. The morphology of the NBeta support and metal dispersion was confirmed by transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray, which revealed that newly synthesized NBeta consisted of individually separated cubical shaped crystals and that the metals were well dispersed in uniform and closely packed ensembles. The nitrate reduction mechanism on NBeta-supported bimetallic catalysts was also identified by inductively coupled plasma mass spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction, and H₂ pulse chemisorption. All catalysts successfully achieved 100% NO₃⁻ and NO₂⁻ removal. Sn–Pd-NBeta showed the highest reduction kinetics (k = 19.09 × 10⁻² min⁻¹) followed by In–Pd-NBeta (k = 2.89 × 10⁻² min⁻¹) and Cu–Pd-NBeta (k = 2.31 × 10⁻² min⁻¹). However, N₂ selectivity was in the order of Cu–Pd-NBeta (92.68%) > In–Pd-NBeta (82.93%) > Sn–Pd-NBeta (80.80%). Characterization of the catalysts revealed that the highest N₂ selectivity of Cu–Pd-NBeta was due to the suppression of a deep hydrogenation capacity of Pd by Cu. The NBeta-supported Cu–Pd, In–Pd, and Sn–Pd catalysts also showed a complete NO₃⁻ reduction with consistent N₂ selectivity in successive and regenerated cycles. The results suggest an eco-friendly catalytic system which can be applied to the optimal ex situ remediation of water and wastewater contaminated with nitrate.